Sunday, 8 November 2015

Do Voles Climb Trees?

A great many species of mammal (and, of course, other creatures) live up trees, spending as much of their lives up in the branches as they can. This is true of the great majority of primates, for instance, which might travel along the ground if they have to get between two trees that aren't very close to one another, but, by and large, would prefer not to descend if they can help it.

The technical term for an animal that climbs regularly is scansorial. Not all scansorial animals are actually arboreal - that is, spending their lives in trees. Many of them live on the ground for much of the time, but will climb trees to, for example, escape from something that's chasing them. So it' perhaps not surprising that scansoriality has evolved many times, and that the physical adaptations associated with climbing are seen in many different, often relatively unrelated, mammals.

Indeed, there are good grounds to suppose that many of the earliest mammals that we know of were at least scansorial, if not fully tree-dwelling. These are the archetypal small shrew-like animals that hid in the trees out of sight the dinosaurs, just waiting for the asteroid to strike so that they could take their turn. (The reality is, of course, more complex than that - but it isn't entirely untrue, either).
The reason that we know these early mammals could climb is because of their anatomical adaptations, which are similar to those we find in climbing animals today. Such adaptations tend to be more obvious in larger scansorial animals, and include such features as elongated fingers, flexible joints, and long tails, and, in more extreme cases, thumbs and prehensile tails. But, while they are sometimes less obvious, we can certainly find them in smaller climbing animals, too - tree squirrels are perhaps, a particularly obvious example.

But what about voles? You might think that one vole looks much like another... and the thing is, you're pretty much right. Voles, at least superficially, do all look fairly, well... vole-like. Yet, in fact, there is much more behavioural and ecological variation among vole species than you might think. But perhaps we ought to take a step back, and ask what a vole actually is.

Voles are an actual group of animals, which is to say, they have a single evolutionary origin and represent a single branch on the great mammalian family trees. Granted, in order for the previous sentence to be true, we do have to accept that lemmings and muskrats are just really big voles, but, other than that, what you think of as a vole probably is one.

Voles look, on the whole, rather like mice, except a bit more rounded, and usually with a shorter tail, smaller eyes and ears, and so on. But, as it happens, they are not members of the mouse family, but rather of the Cricetidae, or "hamster" family. (Not that the latter is a particularly good name, since only about 5% of the species in the family look at all like hamsters, and most look exactly like mice). They first appeared in Asia at the beginning of the Pliocene epoch, before rapidly spreading to colonise much of the Northern Hemisphere.

There are lots and lots of voles - well over a hundred species in total. It's probably fair to say that most of them live on the ground, often digging burrows in the soil. That's likely also how the original voles lived, all those millions of years ago, so any that live in a different way now must have changed since. Perhaps best known are the water voles, which, as their name suggests, have adapted to swimming, and spend a fair proportion of their waking hours doing so.

But there are also voles that spend most of the lives up trees, such as the red tree vole (Aborimus longicaudus) of coastal Oregon, which is pretty much only ever found in Douglas fir trees. And if there are such specialised, arboreal voles, and these voles evolved from creatures that once lived on the ground, there must have been, at least at some point, voles that did a lot of climbing, but hadn't quite decided to dedicate their lives to it. It also hardly be surprising if some of the living species did just that.

But do they? Do sensible, ground-living, voles ever go to the trouble of climbing up a tree... and, if so, why?

Well, yes, they clearly do. Take, for example, the bank vole (Myodes glareolus), a very well-known and common European species. They dig burrows in the ground, but they live in woodland, and it's long been known that they will climb trees from time to time. They are particularly likely to do so in winter, where tree bark is one of the few sources of food they can confidently rely on to be present. In fact, if their population isn't kept in check by native predators, they can be a bit of a nuisance to foresters.

In North America, meanwhile, the southern red-backed vole (Myodes gapperi), a burrowing, ground-dwelling animal found across all but the coldest bits of the US and Canada, has been spotted making nests in trees, even though it's climbing ability does not extend to walking along small branches (its toes apparently not being flexible enough). For that matter, in 2008, one researcher, while trying to find red tree vole nests, was surprised to discover a western red-backed vole (M. californicus) had been nesting almost 11 metres (35 feet) up in a fir tree.

So much for western and southern red-backed voles, what about the northern one? Northern red-back voles (M. rutilus) live in Siberia and Scandinavia in the Old World, and from Alaska to Hudson's Bay in North America, in relatively sparse forests of spruce and birch, and out on the open tundra. We know that they dig burrows in the soil, and the textbooks imply that, while they might clamber into low-lying bushes or the like, they are basically ground-dwelling animals. Yet, while there's nothing in the scientific literature to imply that they are truly climbing animals, that's not what fur-trappers and bird-spotters have said.

While it's true that bird-spotters are not necessarily also expert vole-watchers, it's hard to see what else they could have been seeing. Head further south, and there are plenty of climbing mice - deer-mice, of which there are plenty in North America, love climbing trees, and are actually quite good at it. But once you get to, for example, most parts of Alaska, well, there really shouldn't be anything of that sort up in the trees, so regardless of whether they're really voles, or something else entirely, we'd certainly like to know about them.

And so, a couple of researchers decided to place live animal traps up to 5 metres (15 feet) above ground in spruce trees close to Fairbanks, Alaska. Then, for additional confirmation, they found areas where voles had obviously left tracks in the grass, sneakily placed dabs of peanut butter on nearby tree-trunks, and pointed motion-triggered video cameras at them.

The results? Yes, the fur-trappers had it right: the voles do climb trees. Not particularly high, since none were spotted going above about 2 metres (6 feet), but still, that's not bad for something the size of a vole. In fact, according to the video evidence, they were surprisingly good at it. For instance, when heading back down to the ground, they climb head-downwards, which is the most efficient way of doing it, but not what we humans would normally do. When crossing smaller branches, they effectively use their tail as a counter-balance, and sometimes even leap to grab onto another. Perhaps most surprisingly, they are able to rotate and invert their hindfeet while gripping onto bark, something that squirrels can do, but that - so far as we knew - no vole of any description had the flexibility to manage.

Why do they bother, even when there isn't peanut butter to be had? Food is certainly part of it, and some of the voles were seen feeding on tree lichen. Presumably that's not a great resource, being slow-growing, but at least it should still be there in winter, and, when the winter in question is one in Alaska, you take what you can get. There's also the possibility that they're hiding from predators, although there's no definitive evidence of that, and they don't appear to actually nest up in the trees, which would be the safest option, if they could do it.

Although their climbing ability is better than we thought, they aren't that good at it. But, in a sense, they don't have to be, since, in most parts of their range, there are no similar animals that can climb at all. (There are, of course, squirrels, and so on, but they're much bigger and likely not direct competitors). After a recent re-classification, there are only six known species of red-backed vole, and we now know four of them can climb - the other two live only in China, and have barely been studied. It's quite possible that animals like these were the ancestors of the true tree voles, which are much more comfortable aloft.